Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. Ion dose and ion energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable of up to about 1 mA beam current) are used for lower dose applications.
Ion energy is the dominant parameter used to control junction depth in semiconductor devices. The energy levels of the ions which make up the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand ultra low energy (ULE) levels below one thousand electron volts (1 KeV).
A typical ion implanter comprises three sections or subsystems: (i) an ion source for outputting an ion beam, (ii) a beamline including a mass analysis magnet for mass resolving the ion beam, and (iii) a target chamber which contains the semiconductor wafer or other substrate to be implanted by the ion beam. The continuing trend toward smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. Source/drain junctions in semiconductor devices, for example, require such a high current, low energy application.
Ion sources in ion implanters typically generate an ion beam by ionizing within a source chamber a source gas, a component of which is a desired dopant element, and extracting the ionized source gas in the form of an ion beam. The ionization process is effected by an exciter which may take the form of a thermally heated filament or a radio frequency (RF) antenna. A thermally heated filament thermionically emits high-energy electrons while an RF antenna delivers a high energy RF signal into the source chamber. An example of a high current, low energy ion source is shown in U.S. Pat. No. 5,661,308 to Benveniste, et al., commonly owned by the assignee of the present invention and incorporated by reference as if fully set forth herein.
Either the high-energy electrons (in the case of the thermally heated exciter) or the RF signal (in the case of the RF exciter) is used to impart energy to, and thus ionize, the source gas in the source chamber. Examples of desired dopant elements of which the source gas is comprised include phosphorous (P) or arsenic (As). In an ion source utilizing a thermally heated filament for ionization, the source chamber typically attains temperatures on the order of 1000.degree. C. Temperatures such as this (greater than 400.degree. C.) are usually sufficient to prevent the formation of phosphorous or arsenic deposits on the interior walls of the source chamber. In an ion source utilizing an RF antenna, however, the operational temperatures of the source chamber are typically much lower, on the order of 50.degree. C. Such RF excited sources are often referred to as "cold wall" sources. As a result, the interior walls of the ion source chamber may be contaminated by the formation of phosphorous or arsenic deposits during operation.
Because ion implanters are operated using a variety of process recipes, different types of source gases are run in the source to obtain ion beams comprising the desired dopant ions. If, however, the source chamber walls are contaminated by deposit formation during a previous process recipe (e.g., one involving phosphorous), a later process recipe (e.g., one involving arsenic) may be adversely effected by this cross-contamination. Accordingly, during equipment downtimes between process recipes, it is necessary to remove the accumulated deposit formations from the source chamber interior walls to minimize the risk of contaminating subsequent process recipes.
It is known to clean a plasma treatment chamber using a cleaning gas such as nitrogen trifluoride (NF.sub.3) (see, e.g., U.S. Pat. No. 5,620,526 to Watatani et al.) Such cleaning may be conducted in-situ, without disassembling the ion source. However, in-situ cleaning of an ion source chamber between processes wastes valuable time during which the ion implanter is not being used to make product.
Accordingly, it is an object of the present invention to provide an ion source which provides the capability for in-process cleaning of the source chamber while the ion implanter is operational. It is a further object of the invention to provide a method for achieving such in-process cleaning of an ion source.